The present invention relates to a device for making exposures on discrete portions of a photosensitive surface by means of a light beam, of the kind wherein a support arranged to carry the photosensitive surface, an optical head arranged to direct and to control the light beam emerging from a lamp, traversing symbols in the form of masks distributed on a rotating disc, and travelling further towards said sensitive surface, and means for the mounting and the displacement of the optical head or of the sensitive surface, or of both, are provided.
Devices and apparatuses allowing the exposure of discrete or selected portions of a photosensitive surface, by means of a light beam, are typically used in architecture, in cartography, in the field of topography, etc. These devices actually allow the realization of plans in the just mentioned fields with the aid of information and data stored in a computer. Besides in the cited fields, these devices are also used to carry out plans of machine parts in mechanics starting from coded information stored in a computer. An important and well-known use of such devices is the realization of printed circuits. Such devices are then called photoplotters. Known photoplotters generally comprise a numerically controlled drawing machine wherein a relative movement between the drawing support and a drawing tool is generated according to a coordinate system X-Y. The displacements of the drawing tool are automatically generated by commands, namely date supplied e.g. by a computer. Generally, the drawing tool is a luminous drawing head mounted on the machine. In this case, the drawing support is a photosensitive surface whereupon a light beam is projected. This light beam may traverse certain symbols which are then projected when the machine stops, whereas lines are projected during the displacing movement of the luminous head relative to the sensitive surface. The choice of the symbols and the width of the traced lines, as well as the control of the light intensity of the beam, are automatic.
Presently, several types of photoplotters with luminous heads are available. British patent No. 1,107,981 discloses a device for automatically tracing a line in function of stored date, this device comprising two wagons or equipments which are movable with respect to each other and are moved in the X and Y directions by two servo motors in function of stored date. The movable equipments carry a luminous source to project a luminous pencil onto a photosensitive surface. The apparatus further comprises a diaphragm having a variable opening to act on the dimensions of the image of the light beam, and a prism to rotate the image around an axis comprised in the flux of the light beam. This allows a projected image to be maintained in a preferred orientation relative to the direction of the trace.
French Patent Specification No. 2,061,081 describes an apparatus for the automatic tracing of lines in function of stored data, comprising the coordinates of the points of the lines to be traced, expressed by reference to standard axes. This apparatus is destined for being used in the field of cartography and comprises a light source, means for projecting a light beam on a photosensitive surface, and a diaphragm with a variable opening in the flux of the light beam for controlling the image dimensions. It further comprises movable means variable in function of data, for automatically displacing said light beam and producing the trace of a line to be projected on the photosensitive surface, and means for maintaining the exposition of said trace on the photosensitive surface, at a preset value.
Similar devices and apparatuses are also disclosed in the British Patent Specification No. 1,267,824, French Patent Specification No. 2,098,005, and U.S. Pat. No. 3,330,182.
All these known devices generally comprise relatively complicated optical heads, containing diaphragms, numerous films, prism or mirrors in order to deflect the light beam, and devices for controlling the intensity, the sharpness etc. thus rendering the function of such heads not very certain and, above all, rather slow.